Phelan-McDermid syndrome is a genetic condition caused by the loss or disruption of one copy of the SHANK3 gene, most often due to a deletion at the very tip of chromosome 22 (22q13.3). It is associated with significant developmental delay, absent or severely limited spoken language, low muscle tone, and features that frequently overlap with autism spectrum disorder. The deletions that cause Phelan-McDermid syndrome vary enormously in size, from a sliver of DNA around SHANK3 alone to several million base pairs spanning dozens of neighboring genes — and researchers are increasingly finding that how much genetic material is missing shapes a child's specific pattern of developmental, social, and mental health risk.
How Common Is Phelan-McDermid Syndrome?
A 2026 multisource prevalence study gathered genetic testing data from ten clinical genetics laboratories, research centers, and clinical centers, covering 179,837 individuals who had been tested for autism. Phelan-McDermid syndrome diagnoses accounted for 1% to about 2.5% of autism-tested individuals across these sources. After adjusting for the sensitivity of different genetic tests, the proportion of people with Phelan-McDermid syndrome who do not have an autism diagnosis, and general-population autism rates, the researchers calculated a weighted average prevalence of 13.7 per 100,000 people (95% confidence interval 10.02–18.60), meaning about 1 in 7,300 people in the general population have Phelan-McDermid syndrome. That is substantially higher than earlier estimates, which ranged from 2.5 to 10 per million births.
Everyday Ways to Support Someone With Phelan-McDermid Syndrome
- Ask your care team to use standardized, validated assessment tools. A 2026 review identified 131 validated behavioral assessment tools suited to intellectual disability and neurodevelopmental populations, organized by age group and by domain, including autism screening, adaptive behavior, restricted and repetitive behaviors, challenging behaviors, and mental health screening. Following the 2023 Phelan-McDermid syndrome consensus guidelines on which tools fit which concern can lead to more personalized care than informal observation alone.
- Treat persistent sleep problems as a sensory issue first. In a study of 40 children with Phelan-McDermid syndrome, sensory seeking behavior and oral sensitivity were the strongest statistical predictors of sleep disturbance, individually explaining 19.9% and 34.6% of the variance in sleep problems. Addressing sensory regulation may be as important as standard sleep hygiene advice.
- Pay specific attention to touch, movement, and oral sensitivities. The same study found that sleep difficulties correlated specifically with sensory processing differences in the touch, movement, and oral domains, rather than with sensory sensitivity in general. An occupational therapy evaluation targeting these specific areas may identify concrete starting points.
- Ask your genetics team about the size of the 22q13 deletion. A 2026 study of 63 patients found that the size of a person's chromosome 22q13 deletion was significantly associated with language, motor, social, and psychiatric features. Knowing deletion size will not predict an individual's exact outcome, but it can help a care team anticipate which areas may need closer monitoring.
- Don't assume every concern should be screened with the same tool. Because validated assessment tools are organized by distinct domains, such as autism screening versus mental health screening versus adaptive behavior, choosing the tool that matches the specific concern at hand, rather than relying on a single general-purpose measure, better reflects current consensus guidance.
Does the Size of the Chromosome 22q13 Deletion Predict Mental Health Risk?
A 2026 study of 63 Italian patients with Phelan-McDermid syndrome combined deep clinical phenotyping with precise measurement of each patient's chromosome 22q13 deletion size. Larger deletions were significantly associated with more pronounced difficulties in expressive language (p < 0.001), motor development timing (p < 0.001), gait (p < 0.001), and muscle strength (p < 0.01). Social cognition followed the same pattern: the majority of patients with deletions larger than 3 megabases showed no reciprocal object-exchange gesture and no joint attention, with inconsistent eye contact, while patients with smaller deletions showed a far more even mix of typical and atypical social responses (p < 0.001 to p < 0.05 across these measures). Strikingly, comorbid lifelong bipolar disorder, diagnosed in 12 of the 63 patients overall (19.0%), showed the opposite pattern (p < 0.05): among the 16 patients with the smallest deletions, limited mainly to the SHANK3 gene and its immediate neighbors, 7 (43.8%) had received a bipolar diagnosis, a notably higher rate than among patients with larger deletions. The researchers concluded that SHANK3 loss on its own, without help from neighboring genes, appears to carry much of the risk for bipolar disorder specifically.
The Sensory-Sleep Connection
A separate 2026 study examined the relationship between sensory processing and sleep in 40 children with Phelan-McDermid syndrome, using two standardized parent-report questionnaires: the Children's Sleep Habit Questionnaire and the Child Sensory Profile 2. The researchers found significant correlations between sleep difficulties and three sensory measures: sensory sensitivity, sensory registration, and sensory seeking, with the sleep connection concentrated specifically in the touch, movement, and oral processing domains. A stepwise regression analysis found that sensory seeking and oral sensitivity were each independent, significant predictors of sleep disturbance, together pointing to specific, targetable sensory differences rather than general distress as a driver of poor sleep in this population.
Tips for Parents and Caregivers
- Ask about deletion-size information as part of genetic counseling. A 63-patient study found that deletion size, not diagnosis alone, correlated with specific language, motor, social, and psychiatric features. This information will not predict any one child's future with certainty, but it can help you and your care team set realistic, individualized expectations.
- Don't assume a smaller deletion means fewer mental health concerns. Patients with the smallest deletions, those confined mainly to the SHANK3 gene, showed a higher rate of lifelong bipolar disorder diagnosis (43.8%) than patients with larger deletions in the same study, the reverse of the pattern seen for language, motor, and social measures.
- If sleep is a struggle, start with a sensory evaluation. In a study of 40 children with Phelan-McDermid syndrome, sensory seeking behavior and oral sensitivity were the strongest predictors of sleep disturbance. Consider requesting an occupational therapy assessment of sensory processing before, or alongside, standard sleep interventions.
- Know that Phelan-McDermid syndrome is more common than older estimates suggested. A 2026 multisource analysis of testing data from nearly 180,000 individuals evaluated for autism estimated that about 1 in 7,300 people have Phelan-McDermid syndrome, several times higher than earlier estimates of 2.5 to 10 per million births. If genetic testing has not been offered, it is worth asking whether SHANK3-focused testing is appropriate.
- Choose assessment tools that match the specific concern. Researchers have identified 131 validated behavioral assessment tools organized by domain, including autism screening, adaptive behavior, and mental health screening. Ask your care team to select tools validated for intellectual disability and neurodevelopmental populations specifically, rather than general-population measures.